System and method for communication utilizing time division duplexing

ABSTRACT

An integrated communication system includes a satellite portion and a terrestrial portion. A plurality of timeslots are allocated for transmission and reception of data by the various components of the satellite portion and terrestrial portion. The allocation of timeslots to the satellite portion and the terrestrial portion may be predetermined or dynamically allocated based on traffic loads, time of day, day of week, and the like. Communication may be accomplished on a single frequency with the appropriate allocation of timeslots. The system includes delay compensation to accommodate signal processing delays and signal propagation delays. For example, a satellite may be instructed to terminate transmission prior to the end of its allocated timeslot to permit the signal from the satellite to propagate to its intended destination within the allocated timeslot to thereby avoid spillover into the next timeslot. This avoids interference between various elements of the communication system.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed generally to communications and, moreparticularly, to a system and method for spectrum sharing in acommunication system using time-division duplexing.

2. Description of the Related Art

Telecommunication systems have evolved from simple hard-wired telephonesto complex wireless networks that often include satellite as well asterrestrial components. With wireless systems, allocation of thefrequency spectrum and appropriate use of the allocated spectrum iscritical to satisfactory operation. Complex systems of spectrum sharingand frequency reuse have been developed as one means of sharing thislimited resource with more and more users.

Some communication systems utilize both satellite and terrestrialcomponents. This combined system often refers to the terrestrialcomponents as an ancillary terrestrial component (ATC) communicationsystem. Some proposals have been put forth that allow spectrum sharingby both the satellite portion and the ATC portion of a telecommunicationsystem. That is, the ATC portion of the system reuses the frequencyspectrum currently assigned to satellites.

Unfortunately, these conventional approaches often lead to performancedegradation because it is very difficult to create sufficient distancebetween the satellites and between the ground elements to permitfrequency reuse and still minimize interference. Interference betweenthe satellite portion and the ACT portion of a communication system maylead to unacceptable data error rates and decreased overall systemperformance.

Therefore, it can be appreciated that there is a significant need for asystem and method for frequency spectrum sharing that does not result ininterference and system degradation. The present disclosure describes asystem and method that provides this advantage and others as will beapparent from the following detailed description and accompanyingfigures.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is a diagram of a telecommunications system having a satellitecomponent and a terrestrial component.

FIG. 2 is a functional block diagram of internal components of a userterminal, base station, and/or satellite component of the system of FIG.1.

FIG. 3 is a timing diagram illustrating time division duplexing betweenthe satellite and terrestrial portions of the system of FIG. 1.

FIG. 4A is a diagram illustrating the transmission during allocatedtimeslots.

FIG. 4B is a timing diagram illustrating the allocation of timeslots inthe system of FIG. 4A.

FIG. 5A is a diagram illustrating the transmission during allocatedtimeslots.

FIG. 5B is a timing diagram illustrating the allocation of timeslots inthe system of FIG. 5A.

FIG. 6 is a diagram of a telecommunications system illustrating the useof multiple spot beams from a single satellite.

FIG. 7 is a timing diagram illustrating the timing compensation due topropagation delay used to maintain synchronization between the satelliteand terrestrial portions of the telecommunications system.

FIG. 8 illustrates the use of multiple access methods.

FIG. 9 is a map illustrating an example of satellite coverage inpropagation delays between a sample satellite and two terrestriallocations.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure is directed to a telecommunication system havingboth satellite and terrestrial components. As will be described ingreater detail below, the satellite and terrestrial components sharefrequencies using a time-division duplexing, which will be described ingreater detail below. FIG. 1 is a diagram illustrating sample componentsof a system 100 constructed and operating in accordance with theteachings contained herein. The system 100 has a satellite portion 102and a terrestrial portion 104. FIG. 1 illustrates a satellite 108 and asatellite 110 in Earth orbit. Those skilled in the art will appreciatethat an actual implementation may typically include a larger number ofsatellites. The satellites 108-110 may be in any of known satelliteconfigurations, such as geosynchronous or geo-stationary orbits, mediumEarth orbit (MEO) or low Earth orbit (LEO). General operation of thesatellite portion 102 is known to those skilled in the art and need notbe described in greater detail herein except as to the time divisionduplexing, which will be discussed in greater detail.

The terrestrial portion 104 of the system 100 comprises a terrestrialnetwork 112 and includes a transmission site with a base transceiverstation (BTS) 114 and a transmission site with a BTS 116. Those skilledin the art will appreciate that an actual implementation of the system100 typically includes a large number of transmission sites with eachtransmission site having a number of BTSs. The plurality of BTSs at aparticular transmission site may be configured in a cellular arrangementwith each BTS having multiple antennas (not shown) to provide coveragefor multiple sectors. For the sake of simplicity, the followingdescription will focus on the BTS 114 and the BTS 116 located at theirrespective transmission sites. General operational details of the BTSs114-116 are known to those skilled in the art and need not be describedin greater detail herein except as to the time division duplexingaspect, which will be described in greater detail below.

The BTS 114 and BTS 116 are coupled to the terrestrial network 112 andcontrolled thereby. Most control functions of the terrestrial networkare well known in the art and need not be described in greater detailherein. In one aspect of the system 100, the terrestrial network 112provides timing information to permit the BTS 114 and BTS 116 totransmit data and receive data at predetermined times.

FIG. 1 also illustrates a satellite network 124 comprising a groundstation 126. Those skilled in the art will appreciate that a typicalimplementation of the system 100 may include a plurality of groundstations. The ground station 126 communicates with the satellites108-110 and provides control information thereto. Most of the satellitecontrol functions are well known to those skilled in the art and neednot be described herein, except for the time division duplexing, whichwill be described in greater detail herein.

The system 100 includes a plurality of user terminals. A user terminal(UT) 118 is configured for terrestrial use only. That is, the UT 118 maycommunicate only with the BTSs 114-116. In contrast, the UT 120 is adual-mode terminal and may communicate with either the satellite portion102 or the terrestrial portion 104 of the system 100.

FIG. 1 also illustrates a system controller 128 coupled to theterrestrial network 112 and the satellite network 124. As will bediscussed in greater detail below, the system controller 128 allocatestime between the satellite and terrestrial transmission and receptionand synchronizes transmit and receive times between the various elementsin the system 100.

Also illustrated in FIG. 1 is an external network 130, which is coupledto the terrestrial network 112 and the satellite network 124. Theexternal network 130 is intended to illustrate any other network, suchas a public switched telephone network (PSTN), private network, or eventhe Internet. As those skilled in the art will appreciate, data may berelayed between a UT (e.g., the UT 118) and some destination (e.g., aconventional telephone, internet-connected host computer, etc., notshown) via the external network 130.

FIG. 2 is a functional block diagram illustrating internal componentsused by many elements of the system 100. For example, the satellites108-110, BTSs 114-116, UTs 118-120 and the ground station 126 may allhave the common components illustrated in FIG. 2. Those skilled in theart will appreciate that other common components are included in thesevarious system elements, but need not be shown as their operation iswell understood and not germane to the present discussion. In addition,those skilled in the art will appreciate that other components may beunique to different system elements but need not be shown for the samereason. For example, the satellites 108-110 typically derive power fromsolar panels, while the UTs 118-120 may typically rely on rechargeablebatteries. The BTSs 114-116 and the ground station 126 typically arepowered from external power sources (i.e., AC line-operated), but mayhave backup generators to provide necessary electrical power in theevent of AC power loss. Thus, the different system elements may eachhave different power supplies that all serve the conventional functionof supplying electrical power to the circuitry.

The functional block diagram of FIG. 2 includes a central processingunit (CPU) 140 and a memory 142. In general, the CPU 140 receivesinstructions and data from the memory 142 and executes thoseinstructions. The CPU 140 may be implemented as a conventionalmicroprocessor, microcontroller, programmable gate array (PGA), discretecircuit, application specific integrated circuit (ASIC), or the like.The system 100 is not limited by the specific implementation of the CPU140. Similarly, the memory 142 may be implemented by a number ofdifferent known technologies. The memory 142 may include dynamic memory,static memory, programmable memory, or the like. The system 100 is notlimited by any specific implementation of the memory 142.

FIG. 2 also illustrates a transmitter 144 and a receiver 146. Thetransmitter 144 and receiver 146 may be combined to form a transceiver148. The transmitter 144 and receiver 146 are coupled to an antenna 150.As noted above, the implementation of components, such as the antenna150, depends on the particular element of the system 100. For example,if the functional block diagram of FIG. 2 is describing a user terminal(e.g., the UT 118), the antenna 150 may be a simple omni-directionaldipole antenna or other known terrestrial antenna type. In contrast, ifthe functional block diagram of FIG. 2 is describing a satellite (e.g.,the satellite 108), the antenna 150 may comprise a highly sophisticatedmulti-element electronically-steerable antenna. However, the operationof elements, such as the transmitter 144, receiver 146, and antenna 150is well understood by those skilled in the art as it pertains to each ofthe pieces of the communication system 100.

Those skilled in the art will also appreciate that the transmitter 144and receiver 146 may be configured to operate in a number of differentoperational modes. For example, conventional multiple access techniquesinclude time-division multiple access (TDMA), frequency-divisionmultiple access (FDMA), code-division multiple access (CDMA), andorthogonal frequency-division multiple access (OFDMA). The system 100 isnot limited by any particular form of multiple access. Similarly, thespecific details of the transmitter 144 and receiver 146 are known tothose skilled in the art and need not be described in greater detailherein.

FIG. 2 also illustrates a timing controller 160. As will be discussed ingreater detail below, the timing controller 160 selectively enables thetransmitter 144 and/or receiver 146 to transmit or receive data inrespective designated timeslots. Through the appropriate control oftiming, the system 100 prevents certain multiple system elements frombeing active at the same time, thus avoiding undesirable interference.

The block diagram of FIG. 2 also illustrates a position processor 162,which may be used to determine the present position of the particularsystem element. As will be discussed in greater detail below, the system100 determines the current position of a particular system element(e.g., the UT 118 of FIG. 1) and includes these calculations in theoperation of the timing controller 160. The position processor 162 maybe, by way of example, a global positioning system (GPS) receiver, orthe like. Those skilled in the art will also appreciate that systemelements in a fixed locations, such as the BTS 114, can be provided withposition data during an initialization process. Because the element isfixed in position, dynamic position measurements are unnecessary.

The various components illustrated in FIG. 2 are coupled together by abus system 168. The bus system 168 may include a power bus, address bus,data bus, and the like. For the sake of convenience, these various busesare illustrated in FIG. 2 as the bus system 168.

FIG. 3 illustrates the timing operation of the system 100 in itssimplest form. As illustrated in FIG. 3, the satellite and terrestrialportions of the system 100 transmit and receive data on a singlefrequency. This operation may be thought of as a half-duplex operationin which the same communication channel (i.e., the same frequency) isused to transmit and receive, but not at the same time. In the exampleillustrated in FIG. 3, a first timeslot 202 is allocated to thesatellites in the system 100 (e.g., the satellites 108-110) to transmit.A second timeslot 204 is allocated to an ATC element (e.g., the BTS 114)to transmit, while a third timeslot 206 is allocated to a satellite toreceive data, and a fourth timeslot 208 is allocated to a terrestrialelement to receive data. Applying the example timing diagram of FIG. 3to the system 100 illustrated in FIG. 1, the satellite 108 transmitsduring timeslot 202. One skilled in the art will appreciate that the UT120 is enabled to receive data from the satellite 108 during thetimeslot 202. In timeslot 204, the terrestrial system is activated totransmit. In FIG. 1, the BTS 114 and the BTS 116 are activated totransmit to the UT 118 and the UT 120, respectively. During the timeslot206, the satellite 108 is activated to receive data. During thistimeslot, the UT 120 is activated to transmit data to the satellite 108.In the timeslot 208, the terrestrial system is activated to receivedata. That is, the BTS 114 and the BTS 116 receive data transmitted fromthe UT 118 and the UT 120, respectively. With this form of time-divisionduplexing, only one component of the system 100 is active at any giventime, thus eliminating interference caused by the conventional satelliteand ATC combination system.

The timeslots 202-208 are sequentially allocated for satellite andterrestrial transmission and reception, respectively. However, timeslotscan be allocated on a configured basis (i.e., pre-assigned) ordynamically allocated based on communications traffic load, time of day,day of week, and the like. For example, FIG. 3 also illustratestimeslots 210 and 212, which have both been allocated to a satelliteelement for transmission while timeslots 214 and 216 have both beenallocated to a satellite element to receive data. Similarly, timeslots218 and 220 have been allocated for a terrestrial element to transmitwhile timeslots 222 and 224 have been allocated for a terrestrialelement to receive data. Although the example of FIG. 3 illustratesequal sharing of time slots between the satellite portion 102 and theterrestrial portion 104 of the system 100, such symmetry is notrequired. For example, the system 100 could allocate more time slots toATC elements to accommodate increased communication traffic in theterrestrial portion 104. Thus, the system 100 is not limited to anyspecific allocation of the time slots.

FIGS. 4A and 4B illustrate an example embodiment of the system 100. InFIG. 4A, the UT 118 is only capable of communication with one or more ofthe BTSs (e.g., the BTS 114) in the terrestrial portion 104 of thesystem 100. In this example, the UT 120 is only capable of communicationwith one or more of the satellites (e.g., the satellite 108) of thesatellite portion 102 of the system 100. That is, the UT 120 in theexample of FIGS. 4A and 4B is not a dual-mode device.

In accordance with this example, the system controller 128 assigns afirst timeslot 230, shown in FIG. 4B, for transmissions by theterrestrial network and reception by the UT 118. FIG. 4A illustrates thetransmission of data in timeslots 1-4 using dashed arrows to indicatethe direction of data flow in the communication channels. In a secondsequential timeslot 232, the satellite 108 in enabled for transmissionwhile the UT 120 is enabled to receive data from the satellite. During athird timeslot 234, the UT 118 is enabled to transmit to the terrestrialnetwork (e.g., the BTS 114) while in a fourth timeslot 236, the UT 120is enabled to transmit to the satellite network (e.g., the satellite108). Thus, each element of the system 100 is enabled at the appropriatetime and cannot interfere with the operation of any other element of thesystem 100.

FIGS. 5A and 5B illustrate another example embodiment of the system 100.In FIG. 5A, the UT 118 is only capable of communication with one or moreof the BTSs (e.g., the BTS 114) in the terrestrial portion 104 of thesystem 100. In this example, the UT 120 is a dual-mode device capable ofcommunicating with one or more of the satellites (e.g., the satellite108) in the satellite portion 102 of the system 100 and is furthercapable of communication with one or more of the BTSs (e.g., the BTS116) in the terrestrial portion 104 of the system.

In accordance with this example, the system controller 128 assigns afirst timeslot 240, shown in FIG. 5B, in which the BTS 114 is enabled totransmit data and the UT 118 is enabled to receive data. In oneembodiment, there is sufficient physical distance between the BTS 114and the BTS 116 to permit frequency reuse. In this example, the BTS 116is also enabled for transmission to the UT 120 during the firsttimeslot. Alternatively, if the physical distance between the BTS 114and the BTS 116 is not sufficient to permit frequency reuse, the BTS 116can communicate with the UT 120 on a second predetermined frequency.

During a second timeslot 242, the satellite 108 is enabled to transmitdata and the UT 120 is enabled to receive data. During a third timeslot244, the BTS 114 is enabled to receive data and the UT 118 is enabled totransmit data. The UT 120 may also be enabled to transmit to the BTS 116during the third timeslot 244. As noted above, the transmission from theUT 120 may be on the same frequency as the transmission from the UT 118if there is sufficient separation to permit frequency reuse or on asecond predetermined frequency if there is not sufficient distance topermit frequency reuse. In a fourth timeslot 246, the satellite 108 isenabled to receive data and the UT 120 is enabled to transmit data.

During a fifth timeslot 248, the BTS 114 and the BTS 116 are bothenabled to transmit data and the UT 118 and the UT 120, respectively,are enabled to receive data. During a sixth timeslot 250, the UT 118 andthe UT 120 are enabled to transmit data and the BTS 114 and the BTS 116are enabled to receive data. It should be noted that the UT 118 isenabled to transmit data in two separate timeslots (i.e., the timeslots244 and 250) and enabled to receive data in two separate timeslots(i.e., the timeslots 240 and 248). Thus, each element of the system 100is enabled at the appropriate time and cannot interfere with theoperation of any other element of the system 100.

FIG. 1 merely illustrates a communication pathway between the satellite108 and the UT 120. However, those skilled in the art will recognizethat satellites typically have a sophisticated antenna 150 that employsmultiple electronically-steerable antenna elements to generate aplurality of “spot beams” that provide multiple areas of coverage on thesurface of the earth. Satellites in a GEO configuration may have largespot areas that could cover, by way of example, the entire UnitedStates. In contrast, satellites in a MEO or LEO configuration may havemultiple spot beams that have a much smaller area of coverage.

FIG. 6 illustrates the satellite 108 generating multiple spot beams260-264. The spot beam 260 has an area of coverage 266 that includes theBTS 114 and the UT 118. The spot beam 262 has an area of coverage 268that includes the BTS 116 and the UT 120. The spot beam 264 includes anarea of coverage 270. In the example illustrated in FIG. 6, the area ofcoverage 270 does not include any BTS or UT. However, those skilled inthe art will appreciate that the satellite 108 moves with respect to thesurface of the earth if the satellites are in a MEO or LEOconfiguration. In such a configuration, the area of coverage 266-270 ofthe spot beams 260-264, respectively, will move across the surface ofthe earth as the satellite 108 progresses in its orbit.

With multiple spot beams, it is well known that frequencies can bereused so long as there is sufficient directivity in the antennas of thesatellite 108 and sufficient physical separation between the areas ofcoverage that reuse frequencies. In FIG. 6, the satellite 108 uses afrequency, designated as f₁, for communication in the spot beam 260 andin the spot beam 262. The spot beam 264 intermediate the spot beams 260and 262 may use a different frequency, designated as f₂, to preventinterference with the adjacent coverage areas 266 and 268.Alternatively, the spot beams 260-264 may use one or more frequenciesper spot beam with the same or different allocations of time betweensatellite transmission and reception. In yet another alternativeembodiment, the spot beams 260-264 may use the same or differentfrequency than adjacent spot beams with the same or differentallocations of time between satellite transmission and reception. Byappropriate use of time-division duplexing between the satellite portion102 and the terrestrial portion 104, the system 100 preventsinterference between the various system elements.

Those skilled in the art will also appreciate that communication systemsoften use a first frequency for transmitting data and a second frequencyfor receiving data. In some embodiments the transmit/receive frequenciesare offset pairs of predetermined frequencies assigned as channels. Thehalf duplex operation described herein is also applicable in theseembodiments where timing control is still exerted over the transmitterportions of the various system elements (e.g., the transmitter in the UT120, or the transmitter in the satellite 108) and the receiver portionsof the system (e.g., the receiver portion of the UT 118 or the receiverportion of the BTS 116).

With an understanding of the timing relationship between the variouselements of the system 100, the operation of the system controller 128may now be explained in greater detail. The system controller 128 isresponsible for allocation of time between satellite and terrestrialtransmission and reception on the same frequency in an area covered byboth the satellite portion 102 and the terrestrial portion 104 of thesystem 100. The area controlled by the system controller 128 could be aslarge as a single continent-wide beam or as small as a portion of a cityor county.

The system controller 128 serves to synchronize transmit and receivetimes between the satellite portion 102 and the terrestrial portion 104of the system 100. That is, the system controller 128 allocates thetimeslots to the various elements in both the satellite portion 102 andthe terrestrial portion 104 of the system 100 within the area ofcoverage controlled by the particular system controller. As previouslydiscussed, the allocation of timeslots may be predetermined ordynamically allocated based on factors such as traffic load, time ofday, time of week, geographic area, and the like.

In an exemplary embodiment, the system 100 includes compensation fordelay factors, including propagation delay. Propagation delay can besignificant, particularly with a GEO satellite configuration. Inaddition to propagation delays, signal processing delays, caused byprocesses such as modulation and coding, may introduce additional delaysin the system 100. Those delays may be compensated either inherently inthe system elements or the delay may be added to the propagation delaycalculated by the system 100. The delay compensation process describedbelow will focus on compensation for propagation delays. However, thoseskilled in the art will appreciate that the described techniques mayalso take other processing delays into account when calculatingcompensation times.

FIG. 7 illustrates the operation of the system 100 to controltransmissions between the satellite portion 102 and the terrestrialportion 104 of the system 100. FIG. 7 illustrates the system timing atthe transmitter (e.g., the satellite 108) and at the receiver (e.g., theUT 120). For purposes of discussion of FIG. 7, the transmitter transmitsdata during a timeslot between time t₁ and time t₂. In FIG. 7A, nocompensation is provided. At the transmitter, transmission begins attime t₁ and ends at time t₂, as shown in the top timing diagram of FIG.7A. However, it takes t_(d) seconds for the transmission to propagatefrom the transmitter to the receiver. Thus, data does not begin toarrive at the receiver until t_(d) seconds after the start of theallocated timeslot (i.e., at time t₁+t_(d)), as shown in the bottomtiming diagram of FIG. 7A. This decreases channel utilization, but doesnot cause interference. However, a similar propagation delay occurs atthe end of the allocate timeslot. The transmitter terminatestransmission at time t₂, but the signal continues to arrive at thereceiver until t_(d) seconds after the end of the allocated timeslot(i.e., at time t₂+t_(d)). Thus, the transmission from the transmittercauses interference with the system element assigned the timeslotbetween time t₂ and time t₃.

To overcome this interference source, the system 100 instructs thetransmitter to terminate transmission t_(d) seconds prior to the end ofthe allocated timeslot, as shown in the top timing diagram of FIG. 7B.That is, the transmitter terminates transmission t_(d) seconds prior totime t₂ (i.e., at time t₂−t_(d)). The bottom timing diagram of FIG. 7Billustrates the reception of data. As noted above, with respect the FIG.7A, data does not begin to arrive at the receiver until t_(d) secondsafter the start of the allocated timeslot (i.e., at time t₁+t_(d)). Thisdecreases channel utilization, but does not cause interference. However,the early termination of transmission at the transmitter (i.e., at timet₂−t_(d)) permits the reception of data at the receiver to end by timet₂. Thus, proper synchronization is maintained between the satelliteportion 102 and the terrestrial portion 104 of the system 100.

In an exemplary embodiment, channel utilization may be increased byinstructing the transmitter to begin transmitting data before the startof its allocated timeslot so that data arrives at the receiver in propersynchronization. This is illustrated in FIG. 7C where the top timingdiagram illustrates the transmitter timing. That is, the transmitter isinstructed to begin transmission t_(d) seconds before the start of theallocated timeslot (i.e., at time t₁−t_(d)). The data begins to arriveat the receiver t_(d) seconds later (i.e., at time t₁), as illustratedin the bottom timing diagram of FIG. 7C. Furthermore, the transmitter isinstructed to terminate transmission t_(d) seconds prior to time t₂(i.e., at time t₂−t_(d)). This approach provides greater channelutilization while still maintaining proper synchronization.

In an exemplary embodiment the system controller 128 performs the delaycalculations and determines the appropriate compensation times. Thesystem controller 128 provides the necessary instructions totransmitters within its control area to thereby maintain proper systemsynchronization. However, the delay calculation may be performs by othersystem elements, such as controllers in the terrestrial network 112,controllers in the satellite network 124 or the ground station 126.Further, the calculations may be performed by system elements such as asatellite (e.g., the satellite 108), a BTS (e.g., the BTS 114), or a UT(e.g., the UT 120).

Determination of propagation delay involves a knowledge of therelationship of the location of elements (e.g., the BTS 114) of theterrestrial portion 104 and the earth's surface relative to the locationof the satellite (e.g., the satellite 108) in space.

The following provides a sample time delay calculation to determinerange, which is the distance of the satellite from a point on earthmeasured in latitude and longitude. For a GEO satellite, the range isgiven by the following:${Range} = \sqrt{R^{2} + S^{2} - {2{RS}\quad{Cos}\quad\left( {{User}\quad{{Lat}.}} \right){Cos}\quad\left( {{User}\quad{{Long}.{- {GEO}}}\quad{{Long}.}} \right)}}$

Where R equals the nominal earth radius (approximately 6,378.14kilometers), and S equals the nominal GEO satellite orbit radius(approximately 42,164.57 kilometers). Once the range has beendetermined, the delay can be calculated using the following:Delay=Range/c

Where c is the speed of light. For a GEO satellite at 115 degrees Westlongitude and a terrestrial system at 44 degrees North latitude, 68degrees West longitude, the propagation delay would be 0.131430682seconds. In this example, the system controller 128 would instruct theground station (e.g., the BTS 114) to transmit approximately 0.131430682seconds prior to the allocated timeslot so that the transmission arrivesin synchronization with the appropriate timeslot. The system controller128 also instructs the satellite (e.g., the satellite 108) to terminateits transmission approximately 0.131430682 seconds prior to the end ofits allocated timeslot so that the transmission has propagated to theintended receiver (e.g., the UT 120 in FIG. 4A) within its allocatedtimeslot. In this manner, transmissions from the satellite do not spillover into the next allocated timeslot.

The system controller 128 calculates propagation delay for its controlarea. For example, in FIG. 9 the entire continental United States iscovered by a single satellite beam and may fall under the control of asingle system controller 128. The propagation delay for Key West for aparticular satellite position is approximately 0.121846761 seconds whilethe propagation delay from the same satellite in the same position isapproximately 0.129000522 seconds for a terrestrial component inSeattle.

To assure proper synchronization, the system controller 128 takes thelongest propagation delay into account so that transmissions from asatellite reach the ATC element in Seattle by the end of the allocatedtimeslot for transmission by the satellite. If the system controller 128used the propagation delay for Key West, the transmission would still bearriving at the Seattle ATC element in the next allocated timeslot, thusdisrupting synchronization and causing possible interference. Referringto FIG. 7C, the system controller 128 would instruct the transmitter toterminate transmission t_(dmax) seconds prior to time t₂ (i.e., at timet₂−t_(dmax)) where t_(dmax) is the maximum propagation delay in thecoverage area controlled by the system controller 128. In the examplediscussed above, t_(dmax) is the propagation delay to Seattle.

To improve channel utilization, the system controller 128 instructs thetransmitter to begin transmission prior to the start of the allocatedtimeslot, as discussed above with respect to FIG. 7C. Using thepropagation delays for Key West and Seattle, the system controller 128takes the shortest propagation delay into account so that transmissionsfrom a satellite reach the ATC element in Key West at the start of theallocated timeslot for transmission by the satellite. If the systemcontroller 128 used the propagation delay for Seattle, the transmissionwould arrive at the Key West ATC element prior to the start of allocatedtimeslot, thus disrupting synchronization and causing possibleinterference. Referring to FIG. 7C, the system controller 128 wouldinstruct the transmitter to begin transmission t_(dmin) seconds prior totime t₁ (i.e., at time t₁−t_(dmin)) where t_(dmin) is the minimumpropagation delay in the coverage area controlled by the systemcontroller 128. In the example discussed above, t_(dmin) is thepropagation delay to Key West.

If the satellite in the example of FIG. 9 includes multiple beams formultiple coverage areas (see FIG. 6), the system controller 128 maycalculate the minimum and maximum propagation delays independently foreach of the beam coverage areas and instruct the satellite to terminatetransmission to specific coverage areas based on the maximum propagationdelay for that particular coverage area.

As previously noted, the system 100 accommodates different multipleaccess methods, such as TDMA, CDMA, and OFDMA. The multiple accessmethods can occupy a complete timeslot or a subinterval of a timeslot.FIG. 8 illustrates the allocation of timeslots to different multipleaccess methods. Initial access to the system 100 by a UT (e.g., the UT118) may be accomplished by receiving a control signal in apredetermined timeslot or by receiving a control signal on a controlchannel. Conventional techniques, such as the Aloha reservation,reserved slot, and the like may be used for initial access to the system100. System access operations are well known to those skilled in the artand need not be described in further detail herein.

The foregoing described embodiments depict different componentscontained within, or connected with, different other components. It isto be understood that such depicted architectures are merely exemplary,and that in fact many other architectures can be implemented whichachieve the same functionality. In a conceptual sense, any arrangementof components to achieve the same functionality is effectively“associated” such that the desired functionality is achieved. Hence, anytwo components herein combined to achieve a particular functionality canbe seen as “associated with” each other such that the desiredfunctionality is achieved, irrespective of architectures or intermedialcomponents. Likewise, any two components so associated can also beviewed as being “operably connected”, or “operably coupled”, to eachother to achieve the desired functionality.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art that,based upon the teachings herein, changes and modifications may be madewithout departing from this invention and its broader aspects and,therefore, the appended claims are to encompass within their scope allsuch changes and modifications as are within the true spirit and scopeof this invention. Furthermore, it is to be understood that theinvention is solely defined by the appended claims. It will beunderstood by those within the art that, in general, terms used herein,and especially in the appended claims (e.g., bodies of the appendedclaims) are generally intended as “open” terms (e.g., the term“including” should be interpreted as “including but not limited to,” theterm “having” should be interpreted as “having at least,” the term“includes” should be interpreted as “includes but is not limited to,”etc.). It will be further understood by those within the art that if aspecific number of an introduced claim recitation is intended, such anintent will be explicitly recited in the claim, and in the absence ofsuch recitation no such intent is present. For example, as an aid tounderstanding, the following appended claims may contain usage of theintroductory phrases “at least one” and “one or more” to introduce claimrecitations. However, the use of such phrases should not be construed toimply that the introduction of a claim recitation by the indefinitearticles “a” or “an” limits any particular claim containing suchintroduced claim recitation to inventions containing only one suchrecitation, even when the same claim includes the introductory phrases“one or more” or “at least one” and indefinite articles such as “a” or“an” (e.g., “a” and/or “an” should typically be interpreted to mean “atleast one” or “one or more”); the same holds true for the use ofdefinite articles used to introduce claim recitations. In addition, evenif a specific number of an introduced claim recitation is explicitlyrecited, those skilled in the art will recognize that such recitationshould typically be interpreted to mean at least the recited number(e.g., the bare recitation of “two recitations,” without othermodifiers, typically means at least two recitations, or two or morerecitations).

Accordingly, the invention is not limited except as by the appendedclaims.

1. A communication system having a satellite network portion and aterrestrial network portion, the system comprising: a satellite; asatellite transmitter configured to transmit data on a selectedfrequency; a satellite receiver configured to receive data on theselected frequency; a satellite timing controller to selectively controloperation of the satellite transmitter and the satellite receiver; afirst terrestrial station; a first terrestrial transmitter configured totransmit data on the selected frequency; a first terrestrial receiverconfigured to receive data on the selected frequency; and a firstterrestrial timing controller configured to selectively controloperation of the first terrestrial transmitter and the first terrestrialreceiver, wherein the satellite timing controller and the firstterrestrial timing controller are configured to cooperatively controlthe respective transmitters and receivers to control transmission andreception of data in selected time intervals such that the satellitetransmitter is enabled to transmit data in a first selected timeinterval, the satellite receiver receives data in a second selected timeinterval, the first terrestrial transmitter is enabled to transmit datain a third selected time interval, and the first terrestrial receiverreceives data in a fourth selected time interval.
 2. The system of claim1 wherein the first, second, third, and fourth selected time intervalsare sequential time intervals.
 3. The system of claim 1, furthercomprising a first user terminal capable of communication with the firstterrestrial station only and having a transmitter configured to transmitdata on the selected frequency, a receiver configured to receive data onthe selected frequency, and a first user terminal timing controller, thefirst user terminal timing controller being configured to enable thefirst user terminal receiver to receive data from the first terrestrialstation during the third selected time interval and to enable the firstuser terminal transmitter to transmit data to the first terrestrialstation during the fourth selected time interval.
 4. The system of claim3, further comprising a second user terminal capable of communicationwith the satellite only and having a transmitter configured to transmitdata on the selected frequency, a receiver configured to receive data onthe selected frequency, and a timing controller, the second userterminal timing controller being configured to enable the second userterminal receiver to receive data from the satellite during the firstselected time interval and to enable the second user terminaltransmitter to transmit data to the satellite during the second selectedtime interval.
 5. The system of claim 1, further comprising a dual-modeuser terminal capable of communication with the first terrestrialstation or the satellite and having a transmitter configured to transmitdata on the selected frequency, a receiver configured to receive data onthe selected frequency, and a timing controller, the dual-mode userterminal timing controller being configured to enable the dual-mode userterminal receiver to receive data from the satellite during the firstselected time interval, to enable the dual-mode user terminaltransmitter to transmit data to the satellite during the second selectedtime interval, to enable the dual-mode user terminal receiver to receivedata from the first terrestrial station during the third selected timeinterval and to enable the dual-mode user terminal transmitter totransmit data to the first terrestrial station during the fourthselected time interval.
 6. The system of claim 1, further comprising: asecond terrestrial station having a transmitter configured to transmitdata on the selected frequency, a receiver configured to receive data onthe selected frequency, and a timing controller; a first user terminalhaving a transmitter configured to transmit data on the selectedfrequency, a receiver configured to receive data on the selectedfrequency, and a timing controller; and a second user terminal having atransmitter configured to transmit data on the selected frequency, areceiver configured to receive data on the selected frequency, and atiming controller, the first user terminal timing controller beingconfigured to enable the first user terminal receiver to receive datafrom the first terrestrial station during the third selected timeinterval and to enable the first user terminal transmitter to transmitdata to the first terrestrial station during the fourth selected timeinterval and the second user terminal timing controller being configuredto enable the second user terminal receiver to receive data from thesecond terrestrial station during a fifth selected time interval and toenable the second user terminal transmitter to transmit data to thesecond terrestrial station during a sixth selected time interval.
 7. Thesystem of claim 1, further comprising a controller configured toallocate the selected time intervals to the satellite and theterrestrial station.
 8. The system of claim 7 wherein the controller isconfigured to allocate a plurality of selected time intervals to aselected one of the satellite and the terrestrial station based on aquantity of data to be transmitted.
 9. The system of claim 7 wherein thecontroller is configured to allocate a plurality of selected timeintervals to a selected one of the satellite and the terrestrial stationbased on a predetermined transmission schedule.
 10. The system of claim7 wherein the controller is configured to allocate a majority ofselected time intervals to the satellite.
 11. The system of claim 7wherein the controller is configured to allocate a majority of selectedtime intervals to the terrestrial station.
 12. The system of claim 1wherein the satellite is configured to communicate with a plurality ofcoverage areas on the surface of the earth and the first terrestrialstation is within a first of the plurality of coverage areas, the systemfurther comprising a second terrestrial station within a second of theplurality of coverage areas adjacent to the first of the plurality ofcoverage areas.
 13. The system of claim 12 wherein communications in thefirst of the plurality of coverage areas and the second of the pluralityof coverage areas both use the selected frequency.
 14. The system ofclaim 12 wherein communications in the first of the plurality ofcoverage areas uses a first selected frequency and communications in thesecond of the plurality of coverage areas uses a second selectedfrequency different from the first selected frequency.
 15. The system ofclaim 12, further comprising a third terrestrial station within a thirdof the plurality of coverage areas, the second of the plurality ofcoverage areas being intermediate the first of the plurality of coverageareas and the third of the plurality of coverage areas whereincommunications in the first of the plurality of coverage areas uses afirst selected frequency, communications in the second of the pluralityof coverage areas uses a second selected frequency different from thefirst selected frequency, and communications in the third of theplurality of coverage areas reuses the first selected frequency.
 16. Thesystem of claim 1 wherein the satellite timing controller is configuredto terminate transmission from the satellite transmitter prior to theend of the first selected time interval by an amount of time at leastequal to a propagation delay between the satellite and the firstterrestrial station.
 17. The system of claim 1 wherein the satellitetiming controller is configured to initiate transmission from thesatellite transmitter prior to the start of the first selected timeinterval by an amount of time substantially equal to a propagation delaybetween the satellite and the first terrestrial station.
 18. The systemof claim 1 wherein the first terrestrial timing controller is configuredto terminate transmission from the first terrestrial station transmitterprior to the end of the third selected time interval by an amount oftime at least equal to a propagation delay between the first terrestrialstation and the satellite.
 19. The system of claim 1 wherein the firstterrestrial timing controller is configured to initiate transmissionfrom the first terrestrial station transmitter prior to the start of thethird selected time interval by an amount of time substantially equal toa propagation delay between the first terrestrial station and thesatellite.
 20. A communication system having a satellite network portionand a terrestrial network portion, the system comprising: a satellitehaving a transmitter and receiver configured to communicate on aselected frequency; a first terrestrial station having a transmitter andreceiver configured to communicate on the selected frequency; asatellite controller to selectively control operation of the satellitetransmitter, the satellite controller enabling transmission during afirst selected time interval and terminating transmission at a point intime prior to the end of the first selected time interval wherein thepoint in time prior to the end of the selected time interval is based ona propagation delay between the satellite and the first terrestrialstation; and a first terrestrial controller to selectively controloperation of the first terrestrial station transmitter, the firstterrestrial controller enabling transmission during a second selectedtime interval and terminating transmission at a point in time prior tothe end of the second selected time interval wherein the point in timeprior to the end of the selected time interval is based on a propagationdelay between the first terrestrial station and the satellite.
 21. Thesystem of claim 20 wherein the first terrestrial station is a mobileuser terminal having a transmitter and receiver configured tocommunicate on the selected frequency.
 22. The system of claim 21wherein the user terminal is configured for communication with thesatellite only.
 23. The system of claim 21 wherein the user terminal isconfigured for communication with the satellite or with a secondterrestrial station.
 24. The system of claim 21, further comprising asystem controller configured to determine a propagation delay betweenthe satellite and the first terrestrial station.
 25. The system of claim24 wherein the propagation delay between the satellite and the firstterrestrial station is used by the satellite controller to terminatetransmission at the point in time prior to the end of the selected timeinterval.
 26. A communication system having a satellite network portionand a terrestrial network portion, the system comprising: a satellitehaving a transmitter and receiver configured to transmit and receivedata on a selected frequency; satellite control means for selectivelycontrolling operation of the satellite transmitter and the satellitereceiver; a first terrestrial station having a transmitter and receiverconfigured to transmit and receive data on the selected frequency; firstterrestrial control means for selectively controlling operation of thefirst terrestrial transmitter and the first terrestrial receiver,wherein the satellite control means and the first terrestrial controlmeans cooperatively control the respective transmitters and receivers tocontrol transmission and reception of data in selected time intervalssuch that the satellite transmitter is enabled to transmit data in afirst selected time interval, the satellite receiver receives data in asecond selected time interval, the first terrestrial transmitter isenabled to transmit data in a third selected time interval, and thefirst terrestrial receiver receives data in a fourth selected timeinterval.
 27. The system of claim 26, further comprising a first userterminal capable of communication with the first terrestrial stationonly and having a transmitter configured to transmit data on theselected frequency, a receiver configured to receive data on theselected frequency, and user terminal control means for enabling thefirst user terminal receiver to receive data from the first terrestrialstation during the third selected time interval and for enabling thefirst user terminal transmitter to transmit data to the firstterrestrial station during the fourth selected time interval.
 28. Thesystem of claim 27, further comprising a second user terminal capable ofcommunication with the satellite only and having a transmitterconfigured to transmit data on the selected frequency, a receiverconfigured to receive data on the selected frequency, and second userterminal control means for enabling the second user terminal receiver toreceive data from the satellite during the first selected time intervaland for enabling the second user terminal transmitter to transmit datato the satellite during the second selected time interval.
 29. Thesystem of claim 26, further comprising a dual-mode user terminal capableof communication with the first terrestrial station or the satellite andhaving a transmitter configured to transmit data on the selectedfrequency, a receiver configured to receive data on the selectedfrequency, and dual-mode user terminal control means for enabling thedual-mode user terminal receiver to receive data from the satelliteduring the first selected time interval, for enabling the dual-mode userterminal transmitter to transmit data to the satellite during the secondselected time interval, for enabling the dual-mode user terminalreceiver to receive data from the first terrestrial station during thethird selected time interval and for enabling the dual-mode userterminal transmitter to transmit data to the first terrestrial stationduring the fourth selected time interval.
 30. The system of claim 26,further comprising: a second terrestrial station having a transmitterconfigured to transmit data on the selected frequency, a receiverconfigured to receive data on the selected frequency, and secondterrestrial station control means for controlling the second terrestrialstation transmitter and receiver; a first user terminal having atransmitter configured to transmit data on the selected frequency, areceiver configured to receive data on the selected frequency, and firstuser terminal control means for enabling the first user terminalreceiver to receive data from the first terrestrial station during thethird selected time interval and for enabling the first user terminaltransmitter to transmit data to the first terrestrial station during thefourth selected time interval; and a second user terminal having atransmitter configured to transmit data on the selected frequency, areceiver configured to receive data on the selected frequency, andsecond user terminal control means for enabling the second user terminalreceiver to receive data from the second terrestrial station during afifth selected time interval and for enabling the second user terminaltransmitter to transmit data to the second terrestrial station during asixth selected time interval.
 31. The system of claim 26, furthercomprising allocation control means for allocating the selected timeintervals to the satellite and the terrestrial station.
 32. The systemof claim 31 wherein the allocation control means allocates a pluralityof selected time intervals to a selected one of the satellite and theterrestrial station based on a quantity of data to be transmitted. 33.The system of claim 31 wherein the allocation control means allocates aplurality of selected time intervals to a selected one of the satelliteand the terrestrial station based on a predetermined transmissionschedule.
 34. The system of claim 31 wherein the allocation controlmeans allocates a majority of selected time intervals to the satellite.35. The system of claim 31 wherein the allocation control meansallocates a majority of selected time intervals to the terrestrialstation.
 36. The system of claim 26 wherein the satellite is configuredto communicate with a plurality of coverage areas on the surface of theearth and the first terrestrial station is within a first of theplurality of coverage areas, the system further comprising a secondterrestrial station within a second of the plurality of coverage areasadjacent to the first of the plurality of coverage areas.
 37. The systemof claim 36 wherein communications in the first of the plurality ofcoverage areas and the second of the plurality of coverage areas bothuse the selected frequency.
 38. The system of claim 36 whereincommunications in the first of the plurality of coverage areas uses afirst selected frequency and communications in the second of theplurality of coverage areas uses a second selected frequency differentfrom the first selected frequency.
 39. The system of claim 36, furthercomprising a third terrestrial station within a third of the pluralityof coverage areas, the second of the plurality of coverage areas beingintermediate the first of the plurality of coverage areas and the thirdof the plurality of coverage areas wherein communications in the firstof the plurality of coverage areas uses a first selected frequency,communications in the second of the plurality of coverage areas uses asecond selected frequency different from the first selected frequency,and communications in the third of the plurality of coverage areasreuses the first selected frequency.
 40. The system of claim 26 whereinthe satellite control means comprises means for terminating transmissionfrom the satellite transmitter prior to the end of the first selectedtime interval by an amount of time at least equal to a propagation delaybetween the satellite and the first terrestrial station.
 41. The systemof claim 26 wherein the satellite control means comprises means forinitiating transmission from the satellite transmitter prior to thestart of the first selected time interval by an amount of timesubstantially equal to a propagation delay between the satellite and thefirst terrestrial station.
 42. The system of claim 26 wherein the firstterrestrial control means comprises means for terminating transmissionfrom the first terrestrial station transmitter prior to the end of thethird selected time interval by an amount of time at least equal to apropagation delay between the first terrestrial station and thesatellite.
 43. The system of claim 26 wherein the first terrestrialcontrol means comprises means for initiating transmission from the firstterrestrial station transmitter prior to the start of the third selectedtime interval by an amount of time substantially equal to a propagationdelay between the first terrestrial station and the satellite.
 44. Atiming correction method for use in an integrated terrestrial andsatellite communication system comprising: allocating a plurality oftime slots for use by the communication system, the allocated time slotsbeing used by an earth station for transmission of data and by asatellite for the transmission of data; and controlling transmissionbetween the earth station and the satellite to compensate for apropagation delay time between the satellite and the earth station tothereby synchronize use of the allocated time slots by both the earthstation and the satellite.
 45. The method of claim 44 wherein eachallocated time slot has a start time and an end time and controllingtransmission between the earth station and the satellite comprisesterminating transmission from the satellite prior to the end time of atime slot allocated for transmission of data by the satellite.
 46. Themethod of claim 45 wherein controlling transmission between the earthstation and the satellite comprises terminating transmission from thesatellite prior to the end time by an amount of time substantially equalto the propagation delay time.
 47. The method of claim 44 wherein eachallocated time slot has a start time and an end time and controllingtransmission between the earth station and the satellite comprisesinitiating transmission from the satellite prior to the start time of atime slot allocated for transmission of data by the satellite.
 48. Themethod of claim 47 wherein controlling transmission between the earthstation and the satellite comprises initiating transmission from thesatellite prior to the start time by an amount of time substantiallyequal to the propagation delay time.
 49. The method of claim 44 whereineach allocated time slot has a start time and an end time and whereinthe earth station transmits data directly to a terrestrial receiver inan allocated time slot, the earth station terminating transmission at atime substantially equal to the end time of the allocated time slot. 50.The method of claim 49 wherein the earth station transmits data to theterrestrial receiver via the satellite with the satellite relaying thedata to the terrestrial receiver in an allocated time slot, thesatellite terminating transmission at a time prior to the end time ofthe allocated time slot.
 51. The method of claim 44 wherein eachallocated time slot has a start time and an end time and wherein aterrestrial transceiver transmits data directly to the earth station inan allocated time slot, the terrestrial receiver terminatingtransmission at a time substantially equal to the end time of theallocated time slot.
 52. The method of claim 51 wherein the terrestrialtransceiver transmits data to the earth station via the satellite withthe satellite relaying the data to the earth station in an allocatedtime slot, the satellite terminating transmission at a time prior to theend time of the allocated time slot.
 53. The method of claim 44 whereineach allocated time slot has a start time and an end time andcontrolling transmission between the earth station and the satellitecomprises terminating transmission from the earth station prior to theend time of a time slot allocated for transmission of data by the earthstation.
 54. The method of claim 53 wherein controlling transmissionbetween the earth station and the satellite comprises terminatingtransmission from the earth station prior to the end time by an amountof time substantially equal to the propagation delay time.
 55. Themethod of claim 44 wherein each allocated time slot has a start time andan end time and controlling transmission between the earth station andthe satellite comprises initiating transmission from the earth stationprior to the start time of a time slot allocated for transmission ofdata by the earth station.
 56. The method of claim 55 whereincontrolling transmission between the earth station and the satellitecomprises initiating transmission from the earth station prior to thestart time by an amount of time substantially equal to the propagationdelay time.
 57. The method of claim 44 wherein allocating a plurality oftime slots for use by the communication system comprises allocating afirst portion of the time slots for transmission of data by the earthstation and allocating a second portion of the time slots fortransmission of data by the satellite.
 58. The method of claim 44wherein allocating a plurality of time slots for use by thecommunication system comprises allocating a first portion of the timeslots for reception of data by the earth station and allocating a secondportion of the time slots for reception of data by the satellite. 59.The method of claim 44 wherein allocating a plurality of time slots foruse by the communication system comprises allocating a first portion ofthe time slots for transmission of data by the earth station, allocatinga second portion of the time slots for transmission of data by thesatellite, allocating a third portion of the time slots for reception ofdata by the earth station, and allocating a fourth portion of the timeslots for reception of data by the satellite.
 60. The method of claim 59wherein the allocation of the first, second, third and fourth portionsof the plurality of time slots is based on a quantity of communicationtraffic by at least one terrestrial transmitter.
 61. The method of claim59 wherein the allocation of the first, second, third and fourthportions of the plurality of time slots is based on a quantity ofcommunication traffic by a plurality of terrestrial transmitters. 62.The method of claim 59 wherein the allocation of the first, second,third and fourth portions of the plurality of time slots is based a timeof day, a day of the week or both.
 63. The method of claim 59 whereinthe allocation of the first, second, third and fourth portions of theplurality of time slots is based a coverage area of the earth station, acoverage area of the satellite, or both.
 64. The method of claim 44wherein the transmission of data by the earth station utilizes aselected frequency channel and the transmission of data by the satelliteutilizes the selected frequency channel.